1. Field of the Invention
The instant disclosure relates to a power supply; in particular, to an external power supply and a system connection detection unit applied thereto.
2. Description of Related Art
Enhanced environmental awareness in recent years and global warming have forced saving energy to become one of the major policies of most countries in the world. For the purpose of power saving, the US Department of Energy has developed low-power specifications for all the information and electronic devices. For example, energy efficiency level V1 (Efficiency>88%, Power input<0.21 W at no load) is required for a external power supply. The European Union also has adopted the advanced power saving code CoC tier I (Efficiency>89%, Power input<0.25 W at no load) & tier II (Efficiency>89%, Power input<0.15 W at no load). Therefore, decreasing the power consumption of the external power supply has become a problem to be overcome at present for persons skilled in the art. Under the circumstance of continuing improved energy consumption requirements, the burst-mode control is not sufficient to meet the requirements of approaching extreme low power consumption of the converter. Therefore, the external power supply employing deep sleeping mode has become widely used.
As shown in FIG. 1, the deep sleeping mode makes the power supply unit (PSU) operate intermittently with a very long sleep time, so as to obtain very low input power consumption. The status SA indicates the voltage drop during sleeping, and when the voltage is lower than a certain degree, the status SA changes to the status SB to perform switching the power switches in order to increase the voltage. However, due to the time interval of intermittent work being too long, the output voltage increases greatly and decreases greatly. As such, the output voltage cannot be stable at the required DC voltage for the general system work. When the system is start-up (for example, a computer is power-on) which means the system loading is generated (at the time point T1), a current detection method is usually used to make the power supply start a normal operation mode when detecting the load current draw. Because of the unstable output voltage, if the computer without a battery uses this operation mode, the power supply would not recover the normal operation mode fast enough when the load draw happens, resulting in too low output voltage. As shown in FIG. 1, the voltage drop is quite large after the time point T1. It may cause the abnormal operation. Therefore, conventionally, the aforementioned mode is only applied to (or adapted for) the computer with a battery, such as a notebook computer.